In order to reduce technical effort in wind turbine manufacturing, it is desired to reduce safety redundancies in the design of wind turbine supporting structures.
Limits for reducing such safety redundancies are usually defined by the mechanical stress that affects the wind turbine. A known approach is thus to design the wind turbine structures so as just to withstand the cumulative fatigue loading on the structure throughout its design life when operating under a given set of environmental conditions.
Wind turbines are often operated under conditions in which the fatigue loads are occasionally of such magnitude that if the loads were to be maintained at that magnitude, the fatigue capacity of the wind turbine structure would be exceeded during its entire design lifetime. To meet such situations, it is known in the state of the art to implement load monitoring sensors and to shut down the wind turbine in case of high loads or even overloading.
However, shutting down a wind turbine in such a manner produces undesired times of unavailability, in which the wind turbine does not produce energy. Further, the shutdown process may by itself add to the overall fatigue damage of the wind turbine.
European patent application publication EP 1 674 724 describes a method wherein the position of components of the wind turbine is determined with respect to a fixed reference location with one or more sensors coupled with the wind turbine component. Here, fatigue load analysis is performed based on output signals from the one or more sensors. However, for this method, a plurality of accelerometers is required, which further need to be located in a specific geometrical pattern, for example a number of three accelerometers that are oriented in an orthogonal manner. Further, the solution proposed therein requires complex mathematical processing of the accelerometer signals by performing a double integration in order to provide an approximate displacement figure.
An alternative solution that may avoid the complex calculations of the solution described above is proposed in U.S. Pat. No. 4,297,076. This patent describes a wind turbine that comprises load measurement means (strain gauges) which are coupled to the wind turbine blades for measuring the bending moment on said blades and for providing an output signal representative thereof. Further, it provides adjustment means which are coupled to said load measurement means for varying the pitch of said tip portions in response to said output signal.
In a similar manner, the system presented in United States patent application publication US 2002/0047275 A1 comprises a rotor hub which is provided with measurement means for the mechanical loading at the hub.
However, the use of strain gauges for directly measuring the mechanical loads acting on the wind turbine suffer from the disadvantage that strain gauges, such as bonded or welded strain gauges, require expertise to install and maintain and moreover have only a comparatively short technical lifespan.
Various alternative approaches are known from the state of the art:
In United States patent application publication US 2004/0151577 A1, a wind turbine is described which comprises one or more sensors connected to directly detect deflection of a main shaft flange of the wind turbine from one or more predetermined positions. Further, a signal is generated that corresponds to the detected deflection, and a control circuit is coupled with the sensors to receive the signals and to mitigate a load causing the deflection of the main shaft flange in response to the signals. However, this method has the disadvantage that shaft deflections may be caused by numerous factors, including internal bearing play and flexibility. Further, the correlation between loading and deflection will generally be suboptimal.
United States patent U.S. Pat. No. 4,339,666 describes a blade pitch angle control for a wind turbine generator that utilizes means providing a turbulence factor signal that is indicative of the degree to which the present instantaneous wind velocity may exceed a present average wind velocity. It further proposes signal processing means for providing a load capacity signal as a function of the turbulence measurement. However, the wind speed measurement is taken at a certain point, such as at hub height, so that the point at which the measurement is taken is not indicative of the wind speed that actually acts at the rotor disk.
The system proposed in the aforementioned patent may be combined with the system of UK patent application GB 2 067 247 A, which describes a method in which the instantaneous wind speed is measured at the blade surfaces and where this measurement is used to reduce loads. These methods however do not take into account accumulated fatigue data and will therefore perform more load regulation over the lifetime of the wind turbine than may be needed when talking into account “unused” fatigue capacity.